This invention relates to pager receivers, and more particularly to scanning type pager receivers.
Various arrangements have been proposed for tuning pager receivers. For example, paging receivers produced in the past include a crystal oscillator which generates a reference signal the frequency of which is divided down to a relatively low frequency. These high frequency dividers contribute significantly to shortening the lifetime of the battery which energizes the circuits of the pager receiver. On the other hand, the analog tuned version of this invention uses low current digital to analog converters which significantly improves battery lifetime. Consequently, tuning systems for pager receivers that do not employ a crystal oscillator are desirable.
In one arrangement, the tuning system includes an adjustable bias supply which is coupled to a voltage controlled receiver tuning circuit. The tuning system coarse tunes the pager receiver to the desired receive channel or frequency. Fine tuning is provided by a correction voltage which is coupled from the frequency detector to the adjustable bias circuit. One disadvantage of this type of tuning system is that it must provide a wide enough range on the correction voltage to provide compensation for all component aging and temperature drift. However, providing a frequency correction range sufficiently wide to correct for all component aging and temperature drift could cause the receiver to mistune, pulling in as many as three channels away from the desired tuning channel, for example.
There is another consideration, if the receiver is designed to scan multiple channels for example it may be configured to scan in sequence all 100 channels in the pager FM frequency band. Thus, whenever loss of received signal occurs, the receiver will scan to the next higher system channel immediately. However, if the next system channel were two channels below the present received channel, the pager receiver would have to scan 98 channels before reading the next channel active for the pager receiver. Consequently, messages might be missed during the process of scanning to locate the next available channel for the pager system.
A further possible disadvantage of a scanning pager receiver is that it may not be compatible with tone and voice paging arrangements currently in use. Scanning pager receivers available today are configured to seek channels continuously modulated with system code. Thus, each time the pager receiver seeks an active channel, the pager receiver will scan each of the 100 pager channels in sequence, looking for a system code as each channel is scanned. This could result in loss of a paging signal being transmitted over a channel that is widely separated from the channel at which the pager receiver begins its scanning operation.
Thus, it would be desirable to have a pager receiver that can rapidly determine those channels for a given system on which a detectable signal is present. It would be desirable also to have a pager receiver which minimizes power requirements to enhance battery lifetime for the pager receiver. It would be desirable also to have a scanning paging receiver which is compatible with tone and voice paging systems that are currently available. Moreover, it would be desirable to have a pager receiver including a tuning system which minimizes the time required to seek and lock to a receivable signal channel, thereby minimizing power requirements, and which automatically and continuously adjusts for small variations in frequency tuning thereby compensating for component aging and temperature drift.